Use Cow in {D,Subd}iagnosticMessage.

Each of `{D,Subd}iagnosticMessage::{Str,Eager}` has a comment:
```
// FIXME(davidtwco): can a `Cow<'static, str>` be used here?
```
This commit answers that question in the affirmative. It's not the most
compelling change ever, but it might be worth merging.

This requires changing the `impl<'a> From<&'a str>` impls to `impl
From<&'static str>`, which involves a bunch of knock-on changes that
require/result in call sites being a little more precise about exactly
what kind of string they use to create errors, and not just `&str`. This
will result in fewer unnecessary allocations, though this will not have
any notable perf effects given that these are error paths.

Note that I was lazy within Clippy, using `to_string` in a few places to
preserve the existing string imprecision. I could have used `impl
Into<{D,Subd}iagnosticMessage>` in various places as is done in the
compiler, but that would have required changes to *many* call sites
(mostly changing `&format("...")` to `format!("...")`) which didn't seem
worthwhile.

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -121,10 +121,11 @@ pub fn intrinsic_operation_unsafety(tcx: TyCtxt<'_>, intrinsic_id: DefId) -> hir
     if has_safe_attr != is_in_list {
         tcx.sess.struct_span_err(
             tcx.def_span(intrinsic_id),
-            DiagnosticMessage::Str(format!(
-                    "intrinsic safety mismatch between list of intrinsics within the compiler and core library intrinsics for intrinsic `{}`",
-                    tcx.item_name(intrinsic_id)
-        ))).emit();
+            DiagnosticMessage::from(format!(
+                "intrinsic safety mismatch between list of intrinsics within the compiler and core library intrinsics for intrinsic `{}`",
+                tcx.item_name(intrinsic_id)
+            )
+        )).emit();
     }
 
     is_in_list